1. Field of the Invention
The present invention relates generally to air filters for stationary heating, ventilation, and air conditioning (HVAC) systems, as well as filters used in internal combustion engine induction systems, motor vehicle HVAC systems, and similar environments, and further to surgical and other filtering type masks incorporating air filters.
2. Description of the Related Art
Air filters of various configurations have been developed for use with forced air circulation heating, ventilation, and air conditioning (HVAC) systems in stationary structures, such as homes and office buildings, as well as for use in internal combustion engine induction systems and mobile vehicle HVAC systems. Such filters provide many benefits in prolonging the life of equipment by filtering out contaminants that might otherwise damage delicate parts having close tolerances, as well as making life more comfortable for persons in the filtered air environment.
Such filters have generally been formed of a single filter material or element for the sake of economy. Such single stage or element filters are less costly to manufacture than a multiple stage or multiple element filter, as the step of assembling two or more filter elements together is not required. However, such single stage filters are generally not as efficient as multiple stage filters, as the single homogeneous filter element must be selected to filter the smallest particles, which may damage the system or affect personnel within the HVAC environment. This generally requires relatively fine filtration, which is not necessarily compatible with a filter element selected for use in a relatively dirty or dusty environment. Very fine filter elements in such dirty environments become clogged in relatively short order, while a more coarse filter element allows finer particles through the filter to contaminate the system.
Accordingly, various filters having multiple filter elements have been developed in the past. Such multiple filter elements generally provide a relatively coarse filter element to the inlet side of the filter, and a finer filter element to the outlet side of the filter. In this manner, the finer filter element is not contaminated with coarser particles and tends to capture only the finer particles passing through the system.
An example of such a dual element filter is found in Japanese Patent No. 55-99,315 published on Jul. 29, 1980. According to the drawings and English abstract, this filter comprises a relatively coarser first layer intermittently bonded to a finer second layer across the span of both elements. The filter of the '315 Japanese Patent Publication also uses the same thermoplastic filter material for both of the filter elements, rather than different materials for the two elements.
Another example of a multiple element air filter is illustrated in Japanese Patent Publication No. 64-43,323 published on Feb. 15, 1989. According to the drawings and English abstract, this filter bonds an unspecified nonwoven fabric material to a paper filter material.
Thus, a two-stage air filter solving the aforementioned problems is desired.